1. Field of the Invention
The present invention relates to a white-balance adjusting apparatus and a method thereof for a video camera, and particularly to an improved white-balance adjusting apparatus and a method thereof for a video camera capable of automatically varying a white-balance trace region in accordance with a change of color temperatures of a light source and performing an automatic white-balance adjusting operation.
2. Description of the Conventional Art
Conventionally, when an object is taken for a picture using a camera, every light from the object has its specific color temperature. However, when the image signals of the white color, for example, are displayed on a screen of a television set as a white color, there is a factor of controlling the strength of the primary color signals based on the white color. In this case, we call the factor a white-balance.
Referring to FIG. 1, as shown therein, a conventional white-balance adjusting apparatus for a video camera includes a charge coupled device (CCD) for imaging the light of an object passed through a lens system 1 and for converting the light into electrical signals, a sample/hold, color split and automatic gain control circuit 3 for splitting the electrical signal passed from the CCD 2 into the signals S1 to S3 of yellow Y, green G, and cyan Cy of three channels and for controlling the gains of the three channels, a first operation circuit 4 for converting the signal obtained from the sample/hold, color split and automatic gain control circuit 3 to primary color signals of R, G, B of the three channels, an integration circuit 9 for computing the integration values IR, IG, and IB obtained by integrating the primary color signal outputted from the first operation circuit 4 for a time of a field, a divide circuit 13 for dividing the values integrated at the integration circuit 9 by the values of IR/IG and IG/IB, a gain computation circuit 16 for comparing the values outputted from the divide circuit 4 with a previously stored value and for outputting the gain control signals in accordance with the compared values, a variable gain circuit 5 for amplifying the output level of the primary color signal outputted from the first operation circuit 4 in accordance with the gain control signal obtained at the gain computation circuit 16, a second operation circuit 17 for operating the primary color signal, the gain of which is controlled at the variable gain circuit 5, and for converting the operated signals to the color difference signal R-Y and B-Y, and an encoding circuit 18 for encoding the chrominance signal R-Y and B-Y obtained at the second operation circuit 17 and the luminance signal Y outputted from the external terminal 19.
The detailed operation of the conventional white-balance control apparatus for the video camera will now be explained with reference to FIGS. 1 to 3.
To begin with, when the light image from an object is inputted into the CCD 2, the image is converted into the electrical signals at the CCD 2. The electrical image signal is applied to the sample/hold, color split and automatic gain control circuit 3.
In the sample/hold, color split and automatic gain control circuit 3, the signal outputted from the CCD 2 is split into the signals of Yellow Ye, Green G, and Cyan Cy. Thereafter, the split signals are applied into signals S1 to S3 of three channels of Ye, G, and Cy and then the split channel signal S1 to S3 are controlled to a predetermined gain. Thereafter, the channel signals S1 to S3 are converted to R, G, and B of the primary color signal at the first operation circuit 4 and applied to the variable gain circuit 5 and the integration circuit 9, respectively.
In the integration circuit 9 comprising of first to third integrator 10 to 12, a red color signal R outputted from the first operation circuit 4 is integrated at the first integrator 10 during one field. A green color signal G outputted therefrom is integrated at the second integrator 11 during one field. A blue color signal B outputted therefrom is integrated at the third integrator during one field. Therefore, the integration values IR, IG, and IB in which R, G, and B are integrated by the first to third integrator 10 to 12 are computed and then outputted to the divide circuit 13.
In a first divider 14 of the divide circuit 13, the output signals IR and IG of the first integrator 10 and the second integrator 11 are respectively divided and then the ratio IR/IG is obtained. In addition, in a second divider 15 of the divide circuit 13, the output signals IG and IB of the second integrator 11 and the third integrator 12 are divided and then the ratio IB/IG is obtained. The output signals IR/IG and IB/IG are applied to the gain computation circuit 16.
An integration value ratio IR0/IG0 of an integration value IR0 for a primary red color signal R and an integration value IG0 for a primary green color signal G and an integration value ratio IB0/IG0 of an integration value IB0 for a primary blue color signal B and an integration value IG0 for a primary green color signal G are previously stored in the gain computation circuit 16, so that a curve black body locus CBL is set up based on the above values as shown in FIG. 2. The curve black body locus CBL, as shown in FIG. 2, presents that if one value of the integration value ratio IR/IG obtained at the divide circuit 13 increases, the other value decreases. That is, if the value IR/IG increases, the value IB/IG decreases, and if the value IR/IG decreases, the value IB/IG increases. In addition, if the value IB/IG increases, the corresponding color temperature is high, and if the value IR/IG is high, the corresponding color temperature is low. The white-balance trace regions A1 and A2 are set up at both sides of the curve black body locus CBL, in which if the value IR/IG and IB/IG outputted from the divide circuit 13 are within a range of white-balance trace regions A1 and A2, the white-balance adjustment is performed, whereby the white-balance is automatically performed.
Therefore, in the gain computation circuit 16, whether or not the integration value ratios IR/IG and IB/IG of the primary color signal for the actual image data which are obtained at the divide circuit 13, are within a range of the white-balance trace regions A1 and A2 do judged. If the integration value ratios IR/IG and IB/IG is not fall within a range of the white-balance trace regions A1 and A2, the white-balance adjustment operation is not performed the gains of the first and second variable gain amplifiers 6 and 7 of the variable gain circuit 5, which received the primary color signals R and B from the first operation circuit 4, retain the primary color state. On the contrary, the integration value ratios IR/IG and IB/IG do not fall within a range of the white-balance trace regions A1 and A2, the primary color signals R and B are computed based on the integration ratios IR/IG and IB/IG. The gains computed thereby are applied to the first and third variable amplifiers 6 and 8 of the variable gain circuit 5 as the control signals SGR and SGB, respectively. Here, for example, the gain of the second variable amplifier 7 of the variable gain circuit 5 is previously set as 1.
Thereafter, the control signals SGR and SGB of the gain computation circuit 16 are inputted into the first and third gain amplifiers 6 and 8 of the variable gain circuit 5 to control each of gains thereof. Therefore, in the first through third variable gain amplifiers 6 to 8 of the variable gain circuit 5, the output level of the primary color signals R, G, and B outputted from the first operation circuit 4 establish a relation as "R:G:B=1:1:1" by the control signals SGR and SGB, whereby the white-balance is achieved. In addition, the gain of the gain computation circuit 16 controls the integration value ratio of each of the primary color signals to meet the following condition. EQU (IR/IG)=(IB/IG)=1
In addition, the primary color signals R, G, and B outputted from the variable gain circuit 5 are converted to the color difference signals R-Y and B-Y at the second operation circuit 17 and the converted signals are applied to the encoding circuit 18. The encoding circuit 18 converts the signals applied thereto into the color video signal SVD of a NTSC type using the luminance signal Y outputted from the external terminal 19 and the color difference signals R-Y and B-Y obtained at the second operation circuit 17.
However, generally the integration value ratio 1R/IG and IB/IG of the primary color signals do not obtain the same trace curve between a first trace curve A3 obtained when the light source of the low color temperature moves toward the light source of the high color temperature and a second trace curve A4 obtained when the light source of the high color temperature moves toward the light source of the low color temperature. Instead of the same curve, it creates a hysteresis-like curve. Then the white-balance adjusting operation can be obtained only when the white-balance trace region includes the region of the first trace curve A3 and the second trace curve A4. Therefore, in this case, where the trace region becomes wide, the white-balance adjusting operation cannot be advantageously performed because the white color signal cannot be correctly judged as the white color signal, and the malfunction occurs thereby.